In the manufacture of NPN bipolar transistors at large-scale integration, it is conventional to form a buried layer of n-type material within the p-type substrate, to serve as a subcollector, and to form the base, the emitter and the collector in an epitaxial layer that is deposited on the substrate over the subcollector. A heavily-doped contact collector penetrates through the lightly-doped collector region of the epitaxial layer and is contiguous with the subcollector. The breakdown current path from the base to the contact collector is through the lightly-doped (high resistivity) active collector region of the epitaxial layer and the heavily-doped (low resistivity) subcollector. Therefore, the base-to-collector breakdown voltage BV.sub.CBO of this transistor depends on the doping level of the epitaxial layer, the depth of the subcollector beneath the active base region of the device, and the thickness of the epitaxial layer. Clearly, this presents a problem in the event that it is desired to integrate both high voltage and low voltage devices on the same substrate, since it implies that a thicker epitaxial layer is required for the high voltage device than for the low voltage device, and this in turn implies that additional fabrication steps must be performed.